Generally, a metal pigment formed of metal particles of aluminum, copper, or the like and providing a metallic feel is normally blended into a metallic paint or a metallic ink. In recent years, in order to deal with environmental issues and the like, an aqueous solvent containing water is now being mainly used for blending into a paint or an ink, in place of an organic solvent that has been conventionally used for blending into a paint or an ink. If an untreated metal pigment is blended into an aqueous solvent, problems such as corrosion and the like of the metal pigment occur. Particularly when aluminum particles (aluminum pigment) are blended into an aqueous solvent as metal particles constituting the metal pigment, moisture reacts with aluminum to generate hydrogen gas. For this reason, when an aqueous composition such as an aqueous paint or the like containing an aluminum pigment is stored, there is a risk of explosion of the storage container. Many techniques have been developed in order to solve this problem.
For example, Japanese National Patent Publication No. 01-501631 (PTL 1) proposes a method wherein an aluminum pigment is treated with chromic acid. However, since the method uses a sexavalent chromium compound, its use is difficult in terms of environment and work.
Japanese Patent Laying-Open No. 04-318181 (PTL 2) proposes a method wherein an aluminum pigment is treated with a treating solution containing an oxidizing agent such as molybdic acid or the like, phosphoric acid ions, and alkaline earth metal ions. Japanese Patent Laying-Open No. 09-328629 (PTL 3) discloses an aluminum pigment treated with a polyacid peroxide. Since a coating derived from a polyacid peroxide is dense and has excellent corrosion resistance, an aluminum pigment chemically stable against a water base paint or a varnish for an aqueous ink can be obtained by forming the above coating on the surface of the aluminum pigment. Although this aluminum pigment has excellent chemical stability as described above, because of insufficient dispersibility of the aluminum pigment, there is a problem in that seediness occurs when preparing a coating film, or the resulting external appearance is poor.
Under such circumstances as described above, an aluminum pigment having a coating derived from polymolybdic acid peroxide formed thereon and containing an amine, and an aluminum pigment further provided with an adsorption layer of an organophosphorus compound on that coating, have been found to exhibit excellent water resistance, and also proposed (WO2002/031061 (PTL 4)).
The above-described aluminum pigment having excellent water resistance is obtained as follows: an amine, a solution in which metallic molybdenum is dissolved in hydrogen peroxide solution, and a hydrophilic solvent are added to an aluminum pigment composition containing an organic solvent, these components are mixed and stirred while heating to thereby form an inorganic coating on the aluminum surface, and an organophosphorus compound is added thereto as needed, to provide an aluminum pigment in paste form. This aluminum pigment in paste form can be widely used because it is particularly excellent in water resistance, unlikely to cause aggregation, and excellent in the characteristics of a coating film formed by blending this pigment. This method, however, suffered from difficulty in the control of steps and had danger in terms of work because it requires heating at the time of stirring and mixing for forming a coating derived from a solution in which metallic molybdenum is dissolved in highly reactive hydrogen peroxide solution (polymolybdic acid peroxide) on the aluminum surface, and requires an amine to suppress aggregation caused by a vigorous reaction between polymolybdic acid peroxide and aluminum.
As a method for solving the above-described problem, Japanese Patent Laying-Open No. 2003-301131 (PTL 5) discloses that similar water resistance can be achieved just by blending, in the preparation of a paint, an aluminum pigment dispersion obtained simply by stirring and mixing polymolybdic acid peroxide, an aluminum pigment, water, and a dispersant, without using an aluminium paste in which a coating derived from polymolybdic acid peroxide has been formed in advance on the aluminum surface by heating. This method, however, had a problem in that since polymolybdic acid peroxide is a strongly acidic solution, resin becomes unstable depending on the type and composition of the paint resin, causing aggregation of the aluminum pigment in the paint.